The activation threshold of immune cells is regulated by activating and inhibitory signals received through recognition of self and foreign antigens. Genetic defects that affect activating or inhibitory receptors renders the immune system unable to distinguish between self and non self causing autoimmunity or abnormal response against infectious agents and transformed cells (see, e.g., Walker and Abbas (2002) Nat Rev Immunol. 2:11-19; and Lanier (2003) Curr Opin Immunol. 15:308-314).
Cells of the immune system possess many types of membrane-bound proteins that serve as receptors. The ligands for these receptors may be small molecules, proteins, e.g., cytokines or chemokines, or membrane-bound proteins residing on a separate cell. The occupation of a receptor by its ligand, binding of a receptor by a soluble antibody, cross-linking of like-receptors to each other, and cross-linking of unlike receptors to each other, can result in changes in cellular activity. Some of these events result in “cell activation,” while other events result in “cell inhibition.”
Studies of immune cells and their activation or inhibition have related to: Recruitment of enzymes to the plasma membrane; recruitment of enzymes to “lipid rafts” in the cell membrane (Yang and Reinherz, J. Biol. Chem. 2766, 18775 (2001)), and recruitment of membrane-bound receptors to the plasma membrane. A lipid raft is a region of the plasma membrane with reduced fluidity of the lipid molecules. Cell activation or inhibition also relates to changes in phosphorylation state of receptors; changes in the proliferative state of the cell; calcium fluxes; changes in genetic expression; changes in secretion or in degranulation; differentiation of the cell; changes in the proliferative rate of the cell; changes in cell migration; and changes in chemotaxis. Cell activation may also include the reversal of T cell anergy (see, e.g., Lin, et al., J. Biol. Chem. 273, 19914 (1998); and Sunder-Plassman and Reinherz, J. Biol. Chem. 273, 24249 (1998)).
The question of whether a signaling event, which results in any of the above changes, is activating or inhibiting can be determined on an individual basis. For example, if occupation of an unidentified receptor results in an increases of genetic expression of cytokine mRNA, secretion (or degranulation), release of inflammatory cytokines, phagocytic or lytic activity, the unidentified receptor may be termed an activating receptor. Similarly, if occupation of an unidentified receptor inhibits activity dependent on a known activating receptor, then that unidentified receptor may be termed an inhibiting receptor.
The determination of whether a receptor is activating or inhibiting may be predicted by the polypeptide sequence of the receptor, where the receptor is a protein. Attention has focused on two different motifs: ITIM and ITAM. ITIM stands for immunoreceptor tyrosine-based inhibition motif, while ITAM means immunoreceptor tyrosine-based activation motif. A number of polypeptide receptors bearing one or more ITIM motifs in the cytosolic region of the receptor have been found to be inhibiting, whereas a number of polypeptide receptors bearing one or more ITAM sequences in the cytosolic region have been found to be activating.
Recently, a number of such immunoregulatory receptors like TREMs, MDL-1, FDF03, DCIR, and CD200 have been identified on myeloid cells. FDF03 is also known as Paired Immunoglobulin-Like type 2 Receptor (PILR). The PILR family comprises both inhibitory (PILRα) and activating (PILRβ) isoforms. Both receptors belong to the v-type immunoglobulin superfamily and are expressed on the surface of neutrophils, monocytes, and dendritic cells. PILRb is also present on NK cells and a small population of T cells in both the mouse and human (see, e.g., Shiratori, et al. (2004) J Exp Med. 199:525-33). PIRLα possesses an ITIM in its cytoplasmic domain, whereas the PILRβ transduces activating signals by associating with the ITAM-bearing DAP12 adaptor molecule. The putative ligand for the mouse isoforms was identified to be a CD99-like molecule and more recently, it was observed that the O-glycan sugar chain on CD99 was involved in receptor recognition. Earlier studies in DCs and macrophages have indicated that PILRα can inhibit ITAM-mediated activation signals, a feature common among the ITIM-bearing family of receptors. PILRα exhibited an inhibitory role by blocking intracellular Ca+2 mobilization induced by CD32/FcγRII in FDF03 transfected U937 cells. PILRα has also been identified as a herpes-simplex virus-1 entry co-receptor (see, e.g, Satoh, et al. (2008) Cell 132:935-944.
Innate immunity is distinguished from adaptive immunity in that innate immune cells do not require prior exposure to a particular microbial pathogen or tumor associated antigen to be induced to respond vigorously. In addition, the receptors utilized by innate immune cells are non-polymorphic and generally recognize specific molecular patterns present on microbial pathogens. Thus, innate immune cells provide a first line of defense against pathogens. Typically, an innate immune response is mediated through various myeloid lineage cells, as a first line of defense to pathogens and cancers. Myeloid lineage cells include macrophages, monocytes, dendritic cells, neutrophils, eosinophils, granulocytes, mast cells, basophils, etc. In addition, NK cells are part of innate immunity as well as they express non-polymorphic receptors capable of recognizing altered self; particularly cells that lack MHC class I expression as is the case for many tumor cells. As noted above, FDF03 receptors are expressed on the surface of myeloid lineage cells. In addition, the activating form of FDF03 can be expressed on NK cells. Recent research has explored the concept that innate immune cells may distinguish on microbial pathogen or tumor cell target by processing signal transductions derived from multiple non-polymorphic receptors. Thus one pathogen may trigger receptor interactions based on the various molecules expressed and another pathogen may trigger a different array of receptors, thus triggering differential responses in a qualitative or quantitative manner. Thus, triggering combinations of receptors on innate cells with a combination therapy using two or more agonist molecules may augment the capacity to induce pathogen clearance or tumor cell killing.
A need exists for regulators of immunity, in particular cancer immunity. The present invention fulfills this need by providing methods of regulating cancer immunity with modulators of FDF03 receptors.